Process stabilization is critical for all industrial processes to ensure uniform quality and low production costs. Center lining is a commonly used term for keeping key process variables at desired levels and optimizing them further.
Center lining is not a strictly defined methodology, but there are certain steps that should be followed in a specific order. When you have completed all steps, you can repeat the process.
The five stages of center lining are:
- Identify key variables
- Create variable limits
- Monitor key variables
- Monitor long-term key variables
- Review limits
Process knowledge is required to determine key variables and limits
The first step is to identify the key process variables that affect product quality the most. You need to analyze a long time period and find the times in which quality has met the specified limits and the production rate has been high enough. When you find a suitable time period, you should analyze process data to find key variables. The easiest way to do this is to search the measurements that correlate best with the quality parameters.
The next step is to define the desired levels for the key variables. A standard Statistical Process Control (SPC) method approach is suitable for this: 1) You select the times when the process has worked well, 2) cleanse the data, and then 3) calculate the average and standard deviation values based on that data. Then, for example, three sigma limits can be calculated. Limits must always be dynamic depending on the state of the process.
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Visualization is the key to utilizing SPC limits
It is important that SPC limits and violations are visualized in a user-friendly format. This helps the operator to quickly identify all key variables that are outside the limits. The visualization can consist of gauges or traffic lights to show the current situation in the process, and trends to visualize longer-term stability.
To support operators further, it is recommended that KPI values with a longer perspective are also made available to the operators. One simple solution is to calculate the portion of KPI values that have been within the limits for the previous 24 hours, one week, or some other period.
The system should also help you find out why the key variables are outside the limits. Operators need to know what is causing the limit violation and what they should do next to fix the issue.
Always aim for better
Process development is a continuous task. After the production is stabilized, it is possible to take the next step to tighten the limits to produce even better-quality products consistently. Simultaneously, you can aim to reduce energy consumption and/or reduce raw material costs.
After these steps are taken, you can start the center lining cycle again. Repeating the process is important because optimizing the previous key variables enables you to find more key variables to optimize the process further.
One further aspect to consider are the tools that support the process. Today’s information systems at mills contain a lot of data, and the data that concerns processes and quality is often scattered across several data sources. Without proper data handling methods, center lining is too laborious to implement, and the full potential of process equipment is not utilized. The recommended solution is to use process data diagnostics software that helps you process data effectively and perform the center lining steps efficiently.
This article is based on our white paper published at Papercon in 2019 under the name Smart Paper Machine Analysis Reveals Center Lining Truths. Download the original paper.